Mixing between a reference signal and a data signal is often necessary to extract information about an optical device or network. A probe signal and a reference signal originating from the same source are typically mixed, resulting in fringes that can be detected and used to assess information about the device being probed. In interferometric sensing, a reference signal is mixed with a signal whose phase and/or amplitude is modified by a parameter to be measured. The mixing produces an interference signal, and the amplitude of the interference signal depends on how efficiently the two optical signals mix.
Optical Time-Domain Reflectometry (OTDR) is a widely used tool for identifying problems in large optical networks. OTDR instruments provide measurements of the level of scatter present in a section of fiber, or at a discrete interface over long distances. Optical Frequency Domain Reflectometry (OFDR) may be used to provide data similar to that provided by OTDR over shorter ranges (tens of meters for OFDR instead of 1000's of meters for OTDR) and higher resolutions (tens of microns for OFDR instead of tenths of meters for OTDR). This change in distance scale allows OFDR to be used in applications where the dimensions of interest are centimeters instead of meters such as when optical coupler and switch networks are constructed. For example, OFDR may be used in module-level and sub-module-level diagnostics. The ability of OFDR to measure the complex spectral reflectivity of Rayleigh backscatter as a function of fiber length yields surprising new and very useful results and will be discussed later.
Scatter is the process of redirecting the propagation of light. In an optical fiber, this occurs when light encounters a change in the geometry of the fiber core, or a change in the local index of refraction of a fiber. Scatter generally occurs at any interface such as connectors, poor splices, collimating optics, etc. Typically, light scattered from the forward propagating direction into the backward propagating direction is of primary concern and is called a reflection. Rayleigh scatter, in the context of optical fiber, describes the light scattered in the fiber due to the random nature of the glass structure in and around the fiber core. Although Rayleigh scatter is random in nature, it is fixed because the random pattern of the glass structure is “frozen” into the fiber. Loss is the removal of light from the intended forward propagating mode. Scatter is a form of loss, as is bend radiation and molecular absorption.
Scattered light may be measured and characterized using OFDR. A highly monochromatic beam of light is injected into the optical system or device to be tested. The wavelength/frequency of that light is varied with a time-linear sweep, and the optical signal back-scattered from the optical system is detected by coherently mixing the back-scattered signal with the reference input signal. The beat frequency component of the mixed signal, (corresponding to an interference signal), is measured to determine a position of the back-scattering (reflection) point in the optical system/fiber. The interference signal amplitude also determines a back-scattering factor and an attenuation factor for the reflected light.
U.S. patent application Ser. No. 11/062,740, filed on Jan. 24, 2005, describes ways to use complex data obtained from OFDR measurements of backscatter for an optical device under test (DUT). A fiber segment DUT is identified by itself within a longer fiber DUT or within an optical network DUT that includes multiple fibers coupled to perform one or more functions. OFDR backscatter data, such as a Rayleigh scatter pattern, is used to identify where in a DUT (and for a DUT with plural fibers, in which fiber) a loss occurred and to identify where in a DUT (and for a DUT with plural fibers, in which fiber) a change occurred (e.g., a temperature change resulting in a change in fiber length). Specifically, a detected scatter pattern for an optical fiber is compared to (e.g., cross-correlated with) a reference scatter pattern to determine a characteristic of the optical fiber.
U.S. patent application Ser. No. 11/371,229, filed on Mar. 9, 2006, describes calculating birefringence in a waveguide based on Rayleigh Scatter. Birefringence is related to the “beat length” (which is different from “beat frequency”) of a polarization maintaining (PM) fiber. FIG. 1 conceptually illustrates beat length. A polarization maintaining (PM) optical fiber 1 includes two stress rods 2a and 2b and a waveguide core 3. Light propagating along the core 3 includes two perpendicular polarization vectors, commonly labeled “p” and “s”. These perpendicular polarization vectors correspond to two perpendicular electro-magnetic (EM) fields (only the electric fields are illustrated to simplify the figure and demonstrate the principle). To be a PM fiber, coupling between the two EM fields needs to be minimized so that energy from one polarization/field “mode” is not transferred to the other polarization/field “mode”. That mode coupling decreases as a phase velocity difference between the two polarizations/fields increases.
The stress rods 2a and 2b, which have a different thermal coefficient and index of refraction than the core 3, create a phase velocity difference between the two polarizations/fields. The “fast” electric field shown as the thicker sine wave corresponds to a “fast mode,” and the “slow” electric field shown as the thinner sine wave corresponds to a “slow mode.” The fast mode and slow mode light waves have different phase velocities. The light in the fast mode will have a longer wavelength than the light in the slow mode. As a result, the two electric fields change in phase relative to another as they propagate down the fiber. The two fields start in phase, and then after changing phase by 360 degrees over a certain distance along the fiber, they are back in phase. The distance over which this phase realignment takes place is the “beat length.”
The beat length is a useful parameter to measure for a PM fiber or other optical device because it represents the degree of polarization coupling, (which is usually undesirable), in that PM fiber. A shorter beat length means less mode coupling and a better PM fiber. But beat length should not be confused with a difference in group velocities. As shown in FIG. 2, when two closely spaced wavelengths are present, they form “beat-notes” in each of the modes of the PM fiber corresponding to the envelope waveforms as opposed to the underlying higher frequency waveforms that create the envelopes. The slow and fast envelopes propagate down the fiber at different group velocities. These group velocities can be substantially different from the phase velocities that create the beat length.
Birefringence and beat length are related, and one can be readily calculated from the other. A birefringent material causes different light polarization modes to travel at different speeds through the birefringent material, and birefringence is the degree to which a light wave with two polarizations is split into two unequally reflected or transmitted waves when it travels through a birefringent material. More formally, birefringence, Δn, is given by:nslow−nfast=Δn  (1)where nslow and nfast are the refractive indices for the slow and fast propagation modes, respectively. The beat length d is related to birefringence in accordance with the following:
                    d        =                  λ                                    n              slow                        -                          n              fast                                                          (        2        )            where λ is the nominal operating wavelength (in a vacuum), e.g., a center wavelength of operation of a system where the PM fiber is incorporated or the design wavelength of the fiber.
A Bragg grating can be used to measure birefringence. It is a periodic reflector made up of periodically spaced zones physically formed in or on a section of fiber. The spacing is determined to have a refractive index slightly higher than the fiber core. That spacing reflects a narrow range of wavelengths while transmitting others. FIG. 3 shows conceptually a resonant reflection of a light wave from a Bragg grating. The amplitude of the sum of reflected waves changes linearly with the number of reflections. The frequency of reflection is related to the phase velocity of the transmitted light. The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the velocity at which the phase of any one frequency component of the wave will propagate. In other words, one particular phase of the wave (for example the crest) travels at the phase velocity. (Recall that phase velocity and group velocity are different).
The two polarization modes of a PM fiber have different effective indices of refraction. Thus, they have different propagation constants within the fiber and have different peak reflection wavelengths. Because the electric fields in the two polarization modes have different wavelengths, the same reflector causes the two electric fields to reflect at different light frequencies.
U.S. patent application Ser. No. 11/371,229 describes a way to compute birefringence of a segment of a waveguide at a particular waveguide location by computing the autocorrelation of reflection spectrum associated with a particular location along the waveguide. To perform that calculation, an apparatus measures a complex response of a spectral reflection of the waveguide at a delay corresponding to the particular location along the waveguide. An autocorrelation function is determined, and the birefringence is then calculated based on the distance between side and main autocorrelation peaks.
A limitation of the two approaches described above is the difficulty in distinguishing between two or more physical characteristics or parameters, e.g., strain and temperature, affecting an optical fiber. The ability to distinguish between characteristics like strain and temperature in fiber-optic sensing systems is important to the large-scale success of a fiber sensing technique. In addition, it would be desirable to specifically identify a strain of a particular segment of optical fiber as well as a temperature of that fiber segment. Technology described in this application overcomes this limitation and meets these desirable objectives.
A method and an apparatus are described for analyzing a portion of a polarization maintaining (PM) optical fiber having two polarization states. First and second spectral responses of the PM fiber portion are determined. The second spectral response is determined after a change in one or both of first and second physical characteristics affecting the fiber portion. The first and second spectral responses may be scatter patterns of the PM fiber portion, such as Rayleigh scatter patterns. In a preferred, but non-limiting, example implementation, the spectral responses are determined using Optical Frequency Domain Reflectometry (OFDR). Each polarization state of the PM fiber portion has a corresponding spectral component in the first spectral response. First and second spectral analyses of the PM fiber portion are performed using the first and second spectral responses. Based on those spectral analyses of the PM fiber portion, a first physical characteristic affecting the PM fiber portion is determined that is distinct from a second different physical characteristic affecting the fiber portion. The first and second physical characteristics are typically distributed along the PM fiber and the PM fiber portion is located any where along the PM fiber. Non-limiting example physical characteristics include temperature and strain. An output signal related to the first physical characteristics affecting the fiber portion is provided, e.g., for display, further processing, etc.
In one non-limiting example implementation, the first spectral analysis of the PM fiber portion includes calculating an autocorrelation of the first spectral response and an autocorrelation of the second spectral response, and the second spectral analysis of the PM fiber portion includes calculating a cross-correlation of the first spectral response and the second spectral response. A first autocorrelation peak offset is determined between a central autocorrelation peak and a side autocorrelation peak of the calculated autocorrelation of the first spectral response. A second autocorrelation peak offset is determined between a central autocorrelation peak and a side autocorrelation peak of the calculated autocorrelation of the second spectral response. A difference is then calculated between the first and second autocorrelation peak offsets. Two linear equations are constructed to calculate the first physical characteristic and the second physical characteristic using a matrix of proportionality constants and a vector including the difference between the first and second autocorrelation peak offsets and the cross-correlation of the first spectral response and the second spectral response.
The proportionality constants may, for example, be determined empirically as follows. The PM fiber portion is subjected to a known value of the first physical characteristic with a constant value of the second physical characteristic. A spectral shift is determined to generate a first proportionality constant. An autocorrelation shift is determined to generate a second proportionality constant. The PM fiber portion is subjected to a known value of the second physical characteristic with a constant value of the first physical characteristic. A spectral shift is determined to generate a third proportionality constant, and an autocorrelation shift is determined to generate a fourth proportionality constant.
An advantageous, non-limiting example application of this technology is discriminating and detecting temperature and strain changes affecting a portion of PM fiber. First scatter data of the portion of the PM fiber is determined for first values of temperature and strain. Second scatter data of the portion of the PM fiber is determined for second values of temperature and strain. Preferably, Optical Frequency Domain Reflectometry (OFDR) is used to obtain the scatter pattern data. At least one of the temperature values or the strain values is different between the first and second sets. The first and second scatter data are then used to discriminate between strain and temperature at the portion of the PM fiber and to determine a change in strain or a change in temperature at the portion of the PM fiber. A signal is provided based on the determination of the change in strain or the change in temperature at the portion of the PM fiber. As one example how to use the scatter data, the first and second scatter data are both autocorrelated and cross-correlated. Results from the autocorrelating and the cross-correlating are used to do the discriminating and determining of both a change in strain and a change in temperature at the portion of the PM fiber.